I've used Ubuntu Studio in the past, and it's been pretty good. I'm not sure what bugs you're referring to, but I never noticed any issues when I used it.
One other option is to set the system up yourself. For a multimedia box, usually you'll want a good selection of available software, such as Ardour, Rosegarden, etc. So make sure the distro you're starting from has the software available in its repositories. You'll also want to set up drivers such as Jack (if you're doing audio), which, in my experience, isn't too hard to set up. The toughest part is configuring the kernel's build to be ideal to your system. This usually means making it a realtime kernel and so forth.
It is a bit more work, but in the end you'll get a nice custom built system. Otherwise, I recommend Ubuntu Studio, which I've had good experience with, or 64studio, which is similar. The others I can't say anything about since I know nothing about them.